Digital relativity humidity (RH) sensors can be calibrated by exposing the RH sensor to known RH and temperature environments. An RH sensor can be initially calibrated during manufacturing and re-calibrated periodically during normal use. A common calibration method is to use saturated salt solutions as defined in ASTM E-102, which specifies mixing specific salts with distilled or “pure” water. Saturated salts of specific salts will produce an equilibrium air environment above the saturated salt with a known % RH level, which can then be used to calibrate an RH sensor.
A saturated salt solution within an enclosed volume will create a standard constant relative humidity in the head space above the surface of the saturated salt solution. ASTM E-104 provides procedures for creating saturated salt solutions and a table of reference values for RH values which differ depending on the salt used and the temperature of the solution. These salt solutions are used to create known RH levels for validating the accuracy of an RH probe's integrated RH sensor. However, known devices that utilize such a saturated salt solution all require distilled or pure water.